CALL FOR CONTRIBUTIONS: I am writing a book on "High-Intensity Ultrasonic Technology and Applications" (intended for Marcel Dekker's "Mechanical Engineering Series", edited by Profs. Lynn L. Faulkner and S. Bradford Menkes). This book will focus on the practical application of power (high intensity) ultrasonics, the use of ultrasonic energy to change materials. Contributions are welcome.

THE CAVITATION BUBBLE

ULTRASONICS - continued

[See the photo of a cavitation bubble above,
and the section, More on Cavitation, on page 2]

AL-1V

A POPULARIZED GUIDE TO ULTRASONIC CAVITATION

4-97

(A Non-Technical Explanation of "Cold Boiling")

"Ultrasonic Processing" means "blasting" liquids, usually water, with very intense sound at high frequency, producing very good mixing and powerful chemical and physical
reactions. The process, called "cavitation", is sort of "cold boiling" and results from the creation and collapse of zillions of microscopic bubbles in the liquid.

"Cavitation" or "cold boiling" is easy to understand if you think about what the words "solid", "liquid", and "gas" mean.

A solid is something hard that you can see and touch and hold; its molecules can not move in relation to each other; they are "stuck together".

A liquid is something you can see and touch, but it runs through your fingers if you try to hold it without a cup or a bowl; its molecules are free to move around each
other but they can't move apart. That means that they are "slippery"; they can flow.

A gas is something you can touch, like the wind moving across your hand when you stick it out the window of a moving car, but you can't usually see it and you can't hold
it at all without a closed can or bottle; its molecules are free to move around and together or apart from each other. They can expand or contract without limit.

The definition in physics of a solid is something whose molecules are rigidly bound together in time and space, a liquid is something whose molecules are free to move
around each other at a fixed distance, and a gas is something whose molecules are free to move around each other and to move closer together or further apart.

You know you can bend a solid, like bending a branch or matchstick or toothpick. If you bend it too far, it snaps. If you bend a paper clip back and forth
enough times, you can break it, too; you "fatique" the metal or wear out the bond that holds the molecules together. What you are doing in each case is called
"exceeding the elastic limit"; you are bending it further than it can bend without breaking. With a hammer, you can break a brick or a small stone. With
a big enough hammer or a wrecking ball, you can smash rock or boulders or concrete.

Well, you can break liquids, too! You do it every time you break glass! Glass isn't really a true solid; it is actually a very, VERY, VERY thick
liquid, sort of like a super thick syrup or molasses. If you look carefully at ancient window glass, you can see that it has drooped; it has a bulge toward the
bottom of the pane. That's because it is flowing downhill; gravity is pulling it down even though it's held in the window frame. "Silly Putty" is exactly
the same thing, only not quite as thick; you can see it flow if you wait long enough. But hit it or snap it and it breaks.

Just as you broke the paper clip by bending it back and forth slowly, you can break water (or most other liquids) by jiggling it back and forth, only you have to do it
very quickly. By sticking a vibrating object into water, if you vibrate it far enough (a tiny fraction of an inch) and fast enough (around 10,000 times a second),
you can "fatigue" the water and break the bond between the water molecules. But what does that mean? What was the definition of a gas? Something whose
molecules could move apart. So, if you move water molecules apart, you have a gas, and the gas of water is steam. A steam bubble is normally created by heating
water above the boiling point (212°F or 100°C). But we just did it by fast jiggling, not by heating, so we "cold boiled" the water!

Next, we now have a steam bubble wandering around in a cold liquid, and that just can't be! The steam has to condense (the way steam from a kettle or hot shower
frosts a glass or mirror) and that leaves an empty space behind, a "void" or "cavity", where the steam was. The surrounding water molecules rush in to fill that
cavity; when they reach the center of the cavity, they collide with each other with great force. This is called "cavitation". That makes the molecules bounce
back, creating a "shock wave" which runs outward from the collapsed bubble just like ripples in a pond when you throw in a pebble. The shock wave can wear away
metal; like the edges of an outboard motor propellor. Cavitation was discovered by investigating why propellors wear out.

Where shock waves meet each other, they can cause more steam bubbles to occur and collapse, creating even more cavitation. There, now you're an expert on cavitation!

The action of cavitation induced by ultrasonic energy imparted to a liquid has been introduced in preceding application monographs. This short monograph is
intended to consolidate references to ultrasonic degassing given in related documents into one convenient entity.

Cavitation requires some discontinuity in the liquid, such as gas bubbles or dust motes, about which the bubble forms. A theoretically pure liquid would require
impractically high power levels to initiate cavitation. Ultrasonic degassing initially increases the efficiency of cavitation by removing air bubbles which absorb
acoustic energy and damp sonication.

Ultrasonic degassing is perhaps a slight misnomer, inasmuch as gases are forced both in and out of suspension and solution by ultrasonic action. Degassing in an
ultrasonic field occurs when the rapid vibration of gas bubbles occasioned by the passage of acoustic waves from the radiating surface through the liquid causes adjacent
bubbles to touch and coalesce. As this action progresses with time, bubbles grow to a size sufficient to allow them to rise up through the liquid, against gravity,
until they reach the surface, rise through, and pop (there may be a more elegant scientific term but I am sure the reader will understand what is meant by "pop").

A distinction should be made here between the bubbles which are formed by cavitation and those which occur naturally in the parent liquid or are induced by ultrasonic
action (sparging). Cavitation bubbles, which range in size from infinitesimal to visible (40µm and up) appear only when the radiating surface is activated
and vanish apparently instantaneously when the power is turned off (in actual fact, they vanish within a half cycle or 0.000025 sec. at 20KHz). Naturally-occurring
bubbles of entrapped air or other gases are most evident in freshly-poured hot tap water as a cloudiness or in still water as small bubbles adhering to the undersurface
and the vessel walls. Sparged bubbles, those induced mechanically by external means, such as by ultrasonic action at or near the gas-liquid interface (the surface)
tend to float in the liquid and even cause foam.

Coalescing of either type of bubble is fast and quite visible in water or other clear liquids and is even visible in translucent liquids since it occurs throughout the
bath and so occurs at the walls and surface where it can be viewed. The assumption is made, perhaps unwarrantedly, that the vessel is clear or provided with
viewports or other means of viewing what occurs in the liquid - such visibility is a prerequisite for visual determination. Should visual examination of the
process not be possible, other means of determination, such as neutron radiography, may be employed.

Because a critical factor in successful degassing is that the bubbles grow, rise, and escape through the surface, parameters such as temperature, viscosity, vapor
pressures, and surface tension are also critical. The distance bubbles must travel to reach the surface thus becomes of interest and the process must be designed
to allow for such transit time. In order to provide for transit, the energy may be interrupted periodically, "pulsing" the activity of the radiator. To
further complicate matters, since cavitation causes both sparging and coalescence, the energy level (intensity) must be carefully selected. These are done
empirically; in this area of endeavor, nothing beats cut-and-try experience, and it can be done rapidly and conveniently.

Pulsing is most commonly done by means of a pulsing circuit provided integrally in the generator of the leading brands of ultrasonic processors. These features
generally interrupt the low-voltage control circuitry and allow for variation of pulse interval and pulse length. In degassing, short bursts at low to moderate
intensity, followed by relatively long recovery periods to allow bubbles to rise, suffice. Time ranges might be on the order of a half a second on and ten or
twenty seconds off for liter-batch quantities.

Providing a vacuum above the gas-liquid interface (surface) greatly enhances degassing and requires both a pulse-free (constant pressure) vacuum source and a means of
disposing of the extracted gases if they are in any way environmentally unsafe.

NOTE: Bubbles that appear in the body of the sample liquid during sonication may also represent sonochemical degradation products or high volatiles driven
out by cavitation. If these phenomena are possible, chemical analysis is recomended in critical processes.

WARNING: Flammable or explosive volatiles may be driven out by cavitation and could ignite.
Virtually no sonication devices are explosion-proof and only extreme measures can render them even explosion-resistant.

[Although NOT in the original monograph, reference is added here to the section on Ultrasonics Page 6 re Explosion Resistance] (05 Mar 2010)

In continuous flow operations, some form of standpipe must be provided to prevent pumping pressure from overcoming evacuation pressure, which might otherwise cause the
process liquid to flow out the gas outlet. The height of the standpipe is determined by the weight of the liquid in it, which must exceed the process pressure and
the base of the standpipe must be located directly above the cavitation field. Save such a standpipe, elaborate separation technologies must be employed.

Ultrasonic degassing is a growing area of application, unfortunately held back more by details of mechanical systems (and secrecy) than by the ultrasonic equipment
available. From analysis of dissolved oxygen and carbon dioxide content of soft drinks and wines and spirits to production degassing of process lines, application
of ultrasonic energy holds promise of continued growth in this field.

- - - * - - -

In addition to the considerations noted above for degassing, probe immersion depth must also be taken into account; see Applications Monograph AL-2, "ULTRASONICS AND FINE
PARTICLES", in para. 2.b.

I would also like to add that one of the most convincing demonstrations of the degassing power of ultrasonics has always been to put a cup of carbonated beverage (soda,
pop) in an ultrasonic cleaner or to put a probe into the cup and activate the cleaner or probe; do NOT fill the cup, or be prepared to do a LOT of cleaning
up. The action is very much like vigorously shaking a bottle of soda and releasing the contents; foam in every direction, instantaneously!

Contact the author for more information on the above-noted applications
or other areas in which sonication might prove advantageous.

A new type of horn is now on the market; unlike standard horns which are solid cylinders of metal, the new style is hollow and it "balloons" (or "bulges") outward when
activated away from the transducer crystals, returning to a simple cylindrical form on the return stroke. Thus, the horn radiates radially outward, at right angles
to the longitudinal axis. This enables it to process material in a pipe placed around it, or in a beaker or other vessel in which it might be inserted. Here
is a diagrammatic representation:

DISCLAIMER: The information given here is generic and should NOT be taken as more authoritative than that contained in the instruction manual
which accompanies (or should accompany) the device.

Further, the vast bulk of tips and horns are made of titanium alloy and these instructions apply specifically to that metal, as well as to monel, nickel, and similar
"bell metals" or "bell metal" alloys. Similar effects have been observed in glass, ceramic, and single-crystal radiating faces.
(03 Feb 2012)

Another caveat - these instructions do NOT apply to bonded crystal tips, such as sapphire tips; they must be replaced by the factory.

- - - * - - -

As explained in more detail in the Cavitation Bubble section on Ultrasonics Page 3, the very action of cavitation erodes (and, to some
smaller degree, accelerates corrosion of) the radiating surface of the replaceable tip or solid horn. Performance degrades in proportion to the degree of roughness
of the surface until a point is reached, if the tip does not disntegrate or stop resonating first, at which no significant energy passes into the liquid sample.
Tips which are so heavily eroded (pitted) that the dendritic peaks and valleys are obvious to the unaided eye can trap air or gas bubbles in the valleys (concavities)
and, in effect, stop radiating. Most manufacturers supply tips with a smooth finish (it is a waste of time and money to mirror-polish tips; the finish wlll matte
almost instantly on use). The wear pattern is generally symmetrical on a round or rectangular face, with a small rim of uneroded material remaining around the edge
and the balance of the face becoming gradually darker as material is eroded and the surface roughened. The exception to this is when wear occurs in an abrasive
particulate suspension, in which case the impact of the particles polishes the surface even as it erodes it. Serious erosion usually occurs in concentric rings and
really severe erosion can eat into the dendritic structure of the tip, even perforating through to the back end (the tip/horn joint), in which case the horn itself then
becomes eroded and useless. Further, when erosion progresses so far that pitting extends into the smooth, erosion-free circumferential ring at the edge of the
tip face, the tip (or solid horn) is irreparable and must be replaced.

This illustration shows graphically, if crudely, how dendritic pitting extends into the radiating face, trapping air bubbles (or other gases), which are compressible and
do not propagate cavitation into the liquid (a process very similar to blanketing):
(03 Feb 2012)

Tip life can be best be extended by polishing the tip (the radiating face, only) with an abrasive paper or cloth; do NOT attempt to lathe turn the face - too much
material will be removed. Remember that the tip is part of a finely-tuned resonant body (in effect, a bell) and removing material, by erosion or abrasion, shortens
the length and thus raises the natural resonant frequency. Removing too much material may drive the frequency above that which the generator can accomodate and the
machine may drop out of resonance. Trying to force a machine to resonate above its frequency limit could destroy the driving circuit or even cause failure of the
transducer.

To properly dress a worn tip, do so BEFORE erosion progresses beyond mere matting of the finish. Hold the tip or horn absolutely perpendicular to a piece of
fine carbide grit paper or emery cloth (NOT "sandpaper") placed on a hard, flat work surface and work the tip lightly across the grit in a circular pattern.
Do NOT rock the tip or score it by bearing down heavily; anything that detracts from a smooth, flat finish will cause accelerated erosion. Similarly, do
NOT try to dress a tip by hand polishing with sandpaper or a file. Stop dressing after the matte grey finish is replaced by a finely criss-crossed pattern of
very fine scratch marks.

Above all, do NOT attempt to dress a severely eroded tip! Replace it.

If the machine is old and does not have automatic tuning, or if it is a middle-generation machine that requires nominal tuning, always retune after dressing a tip.